1. Field of the Invention
This invention relates to an oscillation generating apparatus which is suitably used in a testing machine, etc.
2. Prior Art
There are conventional oscillation generating apparatus having a cam mechanism.
A conventional oscillation generating apparatus comprises a crankshaft supported rotatable about its axis, a cam attached to the crankshaft, an oscillating shaft disposed parallel to the crankshaft, an oscillating arm attached to the oscillating shaft perpendicular thereto and extending toward the crankshaft, and a connecting rod attached to a distal end of the oscillating arm and kept in sliding contact with the cam. The connecting rod is pressed against the cam by elastic means so that it keeps in contact with the cam.
The crankshaft is rotated by a motor or an engine etc. at generally a constant speed. The cam is rotated together with the crankshaft. As the cam rotates, the distance of the contact point, where the cam and the connecting rod are in contact, and the axis of the crankshaft varies. Then the connecting rod rotates forward and backward about the oscillating shaft as it slides on the peripheral surface of the cam. Movement of the connecting rod is transmitted to the oscillating shaft through the oscillating arm and the oscillating shaft rotates forward and backward about its axis. In other words, the angle of the oscillating shaft varies as a function of time while the rotational speed of the crankshaft is constant. The angular variation with time of the oscillating shaft is determined by the shape of the cam.
According to the apparatus described above, the connecting rod is kept in contact with the cam by virtue of the elasticity of the elastic means. But, when the rotational speed of the crankshaft becomes high, the connecting rod tends to come apart from the cam because the connecting rod can not move fast enough to trace the shape of the cam. A solution for keeping the connecting rod in contact with the cam is to increase the elastic force pressing the connecting rod to the cam, but this causes other problems such as wear and fatigue of the materials etc. Therefore, maximum rotational speed of the crankshaft is limited so as not to cause disconnection between the cam and the contact rod.
Another problem for the conventional apparatus is that movement of the oscillating shaft involving an abrupt change in angular acceleration is difficult to produce even when the rotational speed of the crankshaft is relatively low because the maximum possible angular acceleration is limited to a low value due to inertia. Therefore, variations in angular acceleration of the conventional oscillating shaft are limited to those containing relatively moderate angular acceleration.